1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an electrophotographic image forming apparatus for forming images using a rotating image bearing member. In addition, the present invention also relates to an image forming method.
2. Discussion of the Background
Image forming apparatuses such as copiers, facsimiles and printers typically perform the following image forming processes.    (1) an electrostatic latent image is formed on an image bearing member (latent image forming process);    (2) the electrostatic latent image is developed with a developer including a toner to form a toner image on the image bearing member (developing process); and    (3) the toner image is transferred onto a receiving material optionally via an intermediate transfer medium (transferring process).
In addition, color image forming apparatuses in which plural color images (such as yellow, magenta, cyan and black images) are overlaid to form a multi-color image or a full color image are well known. Recently, such color image forming apparatuses are required to produce high quality color images at a high speed. Specific examples of such color image forming apparatuses include tandem full color image forming apparatuses using a direct image transfer method in which black (K), yellow (Y), magenta (M) and cyan (C) images formed on the respective image bearing members are transferred onto a receiving material fed by a feeding belt (serving as a moving member) to overlay the color images, resulting in formation of a full color image. It is possible that such direct transfer image forming apparatuses cause a misalignment problem in that the positions of one or more color images formed on a receiving material are deviated from the predetermined positions to an extent such that a user can notify the misalignment of the color images. When such a misalignment problem occurs, the image qualities deteriorate. For example, misalignment of color line or character images causes image quality problems such that line or character images with a secondary color (such as red, blue and green color images), which can be formed by overlaying plural primary color line or character images (such as Y, M and C color images), cannot be formed; the resultant color images look blurred; and a white area is formed around a character image formed on a background with another color. In addition, a banding problem in that an uneven portion like a band is periodically formed on a colored background is also caused.
In addition, tandem full color image forming apparatuses using an intermediate transfer method in which black (K), yellow (Y), magenta (M) and cyan (C) images formed on the respective image bearing members are transferred onto an intermediate transfer belt (serving as a moving member) so as to be overlaid, and the overlaid color images are transferred onto a receiving material also well known. In such intermediate transfer image forming apparatuses also causes a misalignment problem when the positions of one or more color images formed on an intermediate transfer medium are deviated from the predetermined positions.
The misalignment problem is mainly caused by periodical variation in moving speed of the surface of the image bearing members (such as photoreceptor drums). Specifically, when one of the image bearing members is rotated at uneven rotation speed, the position of the color image is deviated from the positions of the other color images. Such periodical variation in moving speed of the surface of an image bearing member is caused by variation in rotation angular speed of a rotation driving force transmitted to the image bearing member such as transmission errors of a driving force transmission device provided on the shaft of an image bearing member (e.g., eccentricity of gears, and accumulative variations of pitches of gears), and transmission errors of coupling provided such that an image bearing member can be detachably attached to a driving force transmission device of an image forming apparatus, (e.g., slanting and eccentricity of the shaft thereof).
In attempting to suppress the periodical variation in moving speed of an image bearing member (i.e., to prevent the misalignment problem), published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 10-78734 proposes an image forming apparatus. The image forming apparatus checks the periodical variation in moving speed of each of photoreceptor drums and adjust the rotation angular speed of each of the photoreceptor drums to prevent occurrence of the periodical moving speed variation problem. Specifically, a detection pattern (i.e., color toner images) is formed on each of the photoreceptor drums and the color toner images are transferred onto an intermediate transfer medium such that the different color toner images are arranged on the intermediate transfer medium in order of K, Y, C and M color. The thus arranged color toner images are sequentially detected with a pattern detection device to determine whether each of the photoreceptors has a periodical (one revolution) variation component of moving speed. When it is determined that a photoreceptor drum has a periodical variation component, the image forming apparatus adjusts the rotation speed of the photoreceptor drum to correct the variation.
The method for correcting the variation in rotation angular speed of a photoreceptor drum is as follows. The detection result of the detection pattern toner images formed on the intermediate transfer medium is influenced by the following two variations in speed. One of the variations is the variation in the moving speed of the surface of the photoreceptor drum. When the moving speed of the photoreceptor drum varies, the positions of electrostatic latent images formed thereon for forming the detection pattern images vary. In addition, when the toner images formed on the photoreceptor drum by developing the electrostatic latent images are transferred to the intermediate transfer medium, the positions of the toner images (i.e., the detection pattern images) on the intermediate transfer medium vary because the moving speed of the photoreceptor drum varies. In this image forming apparatus, the difference in phase (hereinafter referred to as phase difference) between the writing position of an electrostatic latent image (hereinafter referred to as an image writing position) and the transfer position of a toner image (hereinafter referred to as an image transfer position) is about 180°. This angle is hereinafter referred to as a phase difference. In this regard, the phase difference is defined as follows. Let's assume a virtual plane perpendicular to the rotation shaft of the photoreceptor drum. The image writing position (a position SP in FIG. 8) is connected with the center of the rotation shaft to form a first virtual line. In addition, the image transfer position (a position TP in FIG. 8) is also connected with the center of the rotation shaft to form a second virtual line. The phase difference is defined as the angle formed by the first and second virtual lines and is an angle φ in FIG. 8. Then the detection result is multiplied by ½ and phased inverted. The rotation of the photoreceptor drum is controlled using a value obtained by superimposing the correction value on the targeted rotation angular speed of the photoreceptor drum before correction. It is described therein that the periodical variation can be negated by this technique.
The above-mentioned adjustment technique has an assumption such that the phase difference between the image writing position and the image transfer position is about 180°. Therefore, the image forming apparatus is restricted in view of layout of image forming elements.
When the phase difference is different from 180°, a proper correction value cannot be obtained, and thereby control error occurs. It is described in JP-A 10-78734 that the allowance of the phase difference is 180°±45°. Within such a wide allowance, images satisfying the recent requirement for high image quality cannot be produced. For example, when the phase difference is set to 145° in an image forming apparatus, the photoreceptor drum thereof has a radius of 0.20 mm, and is rotated while the moving speed thereof varies by about 0.1% due to eccentricity of a gear driving the photoreceptor drum, the difference in position between the ideal image transfer position of the intermediate transfer medium at which an image is to be transferred and the real image transfer position of the intermediate transfer medium after making the above-mentioned correction is as large as about 12 μm. Hereinafter this difference is referred to as a transfer position difference. When a color image with such a transfer position difference is produced, users can notice misalignment of the image. In recent years, the tolerance level of misalignment of high quality image forming apparatuses is from 40 to 80 μm. Since the variation in moving speed is one of various factors influencing the misalignment of image in an image forming apparatus, the variation of about 12 μm is too large when considering the tolerance level (40 to 80 μm) of the misalignment.
Because of these reasons, a need exists for an image forming apparatus which can reduce the periodical variation in moving speed of the latent image bearing member thereof without restricting the phase difference.